Clear active tree scopes on StyleEngine::didDetach().
clearResolver() is not only called on didDetach(). Make it private and
name it clearResolvers to reflect that it clears scoped resolvers as
well. The comments related to style resolver reconstructruction is
removed as that does not happen anymore.
Clearing m_treeBoundaryCrossingScopes is moved into didDetach()
which was a more natural place.
Clear active and dirty tree scopes in didDetach to not unnecessarily
hang on to any memory associated with them.
These changes were done investigating issue 675533, but won't
necessarily fix anything for that issue.